A discharge gas of a vehicle may include harmful substances such as carbon monoxide (CO), nitrogen oxides (NOx), hydrocarbon (HC), and the like. Among the above three substances which are generated in a combustion process, the nitrogen oxides exist in an inverse relation with respect to the carbon monoxide and the hydrocarbon.
That is, when the carbon monoxide and hydrocarbon are most significantly reduced in a practical output range, the nitrogen oxides are most significantly increased. The nitrogen oxides are more increased as complete combustion of fuel is accomplished, that is, as the temperature of the engine is increased.
Accordingly, various technologies have been developed to reduce the discharge gas, as an allowable amount of portions of the discharge gas, such as the nitrogen oxides, are regulated by a related Act. One of the technologies is an exhaust gas recirculation apparatus (hereinafter ‘EGR’).
The EGR supplies part of combusted gas (EGR gas) with a mixed gas inhaled to a combustion chamber while maintaining a mixing ratio to a theoretical air-fuel ratio in order to reduce the amount of the generation of nitrogen oxides without significantly increasing other harmful substances. The EGR is a device that increases the heat capacity of the combustion gas to lower the temperature of a flame.
In more detail, the EGR is an apparatus for recycling the exhaust gas among the discharge gas to an intake system and lowering the combustion temperature in the cylinder to suppress the generation of nitrogen oxides. This EGR is a means for reducing nitrogen oxides (NOx) in the exhaust gas, and refers to an apparatus for turning a portion of the exhaust gas back to the intake system and lowering the highest temperature when the mixed gas is burned to reduce the generation of nitrogen oxides (NOx).
An EGR cooler among the configuration of the EGR is a kind of a heat exchanger which cools the high temperature exhaust gas using cooling water of the engine as a refrigerant. The EGR cooler may be mounted in, or on, the outside of the cylinder block or may be mounted in, or on, the outside of the engine in order to cool the EGR cooler.